1. Field of the Invention
The present invention relates to a light-emitting device, and, more particularly, to a light-emitting device having a plurality of emission layers.
2. Description of the Background Art
In recent years, displays using organic electroluminescent devices (hereinafter referred to as “organic EL devices”) as flat-panel display devices have been developed to find applications in a wide variety of information equipment. The organic EL devices are expected to provide flat-panel displays that consume less power than CRTs conventionally and generally used in the art, and are also expected to provide nonpolluting (i.e., mercury-free) illumination devices, which will replace fluorescent lamps, and the like.
In the organic EL device, electrons and holes are injected from an electron injecting electrode and a hole injecting electrode, respectively, into an emission layer, so that the electrons and the holes are recombined in the emission layer so as to bring organic molecules into the excited state. The organic EL device then emits fluorescent light when the excited organic molecules return to the ground state. As known in the art, the organic EL device has a multi-layer structure including layers formed of, for example, a material capable of transporting electrons, a material capable of transporting holes, and a luminescent material, so as to provide an improved luminous efficiency.
In recent years, an organic EL device including a plurality of emission layers adapted to emit different wavelengths of light has been proposed in, for example, Japanese Patent No. 3287344. The organic EL device disclosed in this patent includes a first emission layer formed by doping a host material with a first fluorescent material (or dopant material) that emits orange light, and a second emission layer formed by doping a host material with a second fluorescent material (or dopant material) that emits blue light. The orange light and blue light emitted by the respective emission layers are mixed together to produce a white emission.
Recently, an improvement of the luminous efficiency of the organic EL device has been desired to make the device suitable for practical use. In the case where white light is changed or converted through color filters so as to produce full-color emissions, in particular, the luminous efficiency needs to be further improved in view of an optical loss caused by the color filters.
In the organic EL device disclosed in the above-mentioned Japanese Patent No. 3287344, however, each of the first emission layer that produces an orange emission and the second emission layer that produces a blue emission contains a luminescent dopant (i.e., a fluorescent material) as a single dopant, and it is thus difficult to enhance the luminous efficiency to an even higher level. If the organic EL device has a low luminous efficiency, a large amount of current is required to pass through the device, and the device may degrade earlier than expected, resulting in a reduction in the reliability (or lifetime) of the device. Thus, when it is difficult to improve the luminous efficiency, it is also difficult to improve the reliability (or device lifetime).